I've owned the Sigma 35, 2 copies of the 35/2 IS, and 2 copies of the 35L.
I would take exception with this article stating the 35/2 IS is "sharper" than the 35L. Wide open, yes. But the 35L is much sharper @ F/2 especially in the center and vignettes much less.
As far as 'clinical" sharpness, yep, the Sigma has that, along with a brownish tint that makes up the character of the lens. The 35L IMO just has the best bokeh, color, contrast and rendering. If I took 100 shots with the 35L and the 35/2 IS, the latter might result in 100 really good photos, but the former might have 8-10 amazing shots. The 35L is still the king of the 35's.
Sigma = sharpest
Canon 35/2 IS= light, IS, better AF accuracy than the Sigma, plenty sharp enough.
Canon 35L = best build, fastest and most accurate AF of the three, best color/contrast/bokeh. Really sharp around F/2 and beyond, and F/1.4 is usable.